> I'm not sure how to resolve these other than to say "put commas wherever you like, we'll just ignore them".
Very dangerous. I know you were speaking to "within parens" or some other constraint, but it's very easy for authors to discard that constraint (just like you did in your quote) and chaos ensues.
Background-size: 10px, 20px;
Background-size: 10px 20px;
These are very different.